Thank you for the extensive response! I did miss part of your point about density in that you were talking more about dynamic and multi-use zoning and not necessarily about a need to include verticality as well. I think you covered this in a paragraph I under-focused on in my first read when you said that great cities became denser due to constraints that we often no longer necessarily deal with, but should still emulate on empirical evidence. The underlying point there, which you were pretty clear on, being that they were “forced” to develop, as opposed to today where we make such development illegal.
I’m not familiar with Jan Gehl, but I’ve read a few of Jane Jacobs’ books and agree with hers and your assessment of the poor urbanism of tower blocks and the overall failures of single-use zoning.
As to monocultures, If my memory is correct, Jacobs’ take on the dangers of are well covered in Economy of Cities. I’m also a big fan of Taleb, Antifragile, and his works on systems that not only are resistant to shocks, but benefit from them. Strong Towns appears to have done an entire series linking that book to urbanism.
I think we’re in broad agreement that allowing for human-scale development and self-organizing, chaotic, organic growth is a necessary requirement for a great city, but it seems I’m some amount further from saying it’s the seed. There are too many cities that are hollowed out because of some factor(s) that makes their existing mixed-use, walkable centers no longer economically viable. Often the circumstances seem to be an economic monoculture, or a poorly-placed urban renewal highway program, but the fact remains many of these places may never come back, and may only survive as far less intensive, more cheaply built places. Graceful decline is not something we’re good at, and it seems silly we have to worry about that with superheated megacities looking to release pressure, yet here were are.
I want to believe that our smaller cities that want to grow and change will be able to if they provide the right environment, but I am not hopeful they will be able to overcome the inertia of the megacities. Perhaps my myopic focus on the legacy cities of the Northeast and Midwest is the issue: New York might be a hugely successful and expensive city, but that doesn’t mean that Newark or Paterson or Bridgeport should be the next success story even if they did all the right things, not when a place like Austin or Boulder is also in the mix.